1. Field of the Invention
The present invention relates to a sheet ejecting device, and more particularly to a sheet ejecting device that is suited to be employed in an image forming apparatus such as an electrophotographic copying machine or a printer, or in an after-processing device such as a finisher connected to the image forming apparatus.
2. Description of Related Art
When a printed sheet is ejected from an image forming apparatus onto a sheet tray through a pair of ejection rollers, if the sheet is soft, trouble will occur. Specifically, the leading part of the soft sheet bends downward, and the sheet interferes with sheets already ejected and stacked on the tray. Thereby, the sheets stacked on the tray will be out of alignment.
In order to avoid this trouble, as disclosed by JP58-38641A and JP2005-263418A, conventionally, a currently ejected sheet is curved in a direction perpendicular to the sheet ejecting direction by use of a spring, an elastic member, a roller or the like so that the leading edge of the currently ejected sheet will be in contact with a stack of sheets on the tray at a point farther from the pair of ejection rollers. Thereby, the leading part of the currently ejected sheet is prevented from interfering with the stack of sheets on the tray and from pushing the stack of sheets on the tray out of alignment. However, since the currently ejected sheet is curved and strengthened evenly from the leading edge to the trailing edge, the sheet may jump out of the pair of ejection rollers with great force, and consequently, the sheet may push the stack of sheets on the tray out of alignment.
FIG. 17a shows a case of ejecting a sheet S onto a sheet tray 110 via a pair of ejection rollers 101 while not strengthening the sheet S. If the sheet S is very soft, the leading part of the sheet S bends downward after coming out of the ejection rollers 101 and interferes with a stack of sheets S′ at a position near the ejection rollers 101, and the stack of sheets S′ is pushed.
As FIG. 17b shows, when the sheet S is strengthened and ejected, the leading edge of the sheet S comes into contact with the stack of sheets S′ at a point farther from the ejection rollers 101, and the interference between the sheet S and the stack of sheets S′ is eased. Thereby, the sheet S is ejected onto the sheet tray 110 in alignment with the stack of sheets S′. FIG. 17c shows a case of ejecting a sheet S1 of a relatively large size while strengthening the sheet S1 from the leading edge to the trailing edge evenly. In this case, before the trailing edge of the sheet S1 comes out of the ejection rollers 101, the strengthened sheet S1 becomes soft again, and it does not occur that the sheet S1 is tense between the ejection rollers 101 and the stack of sheets S′. Thereby, the sheet S1 is ejected onto the stack of sheets S′ smoothly, and there is no possibility that the sheet S1 and the stack of sheets S′ on the sheet tray 110 will be out of alignment.
FIG. 17d shows a case of ejecting a sheet S2 of a relatively small size while strengthening the sheet S2. In this case, the leading edge of the strengthened sheet S2 comes into contact with the stack of sheets S′, and while the sheet S2 keeps strong, the trailing edge of the sheet S2 comes out of the ejection rollers 101. Therefore, the sheet S2 is tense between the ejection rollers 101 and the stack of sheets S′, and the sheet S2 comes out of the ejection rollers 101 with great force. Thereby, the sheet S2 cannot be in alignment with the stack of sheets S′.